In reading over the article on gameplay differences in Mass Effect 2 as compared to the first game on the ME wiki, I noticed this section under Advancement, emphasis mine:
The new level cap is 30. Further advancing powers is no longer a simple point per power advancement system. Powers now cost an increasing number of points to advance and offer a specialization choice when maxed out. Since you can only max out five powers, this greatly affects play style of the characters. As such it is now possible to reset your advancement and reset your character in-game.
When can I respec? How often? Does it cost ingame currency (credits/resources)? Do I need to complete a certain mission first?
Best Answer
To reassign points to your powers, you'll need to use the Retrain Powers upgrade in the Research Terminal, which can be found in the Research Lab on the second deck of the Normandy. In the Research Terminal menu, it's located under the Prototypes research category with the following description:
This upgrade becomes available after you complete the mission on Horizon, and costs 2500 Element Zero per use.
As yx. mentions, you can also access this upgrade at the Research Terminal in the Shadow Broker base after completing the mission associated with the Shadow Broker DLC. The terminal will be to the right and up the stairs on subsequent revisits, and operates the same as the one on the Normandy.
MBraedley's note in the comments is also correct. In the Shadow Broker's base, to the left, you'll find the Advanced Training* console. This console allows you to redistribute the ability points of your squadmates much like Retrain Powers can be used on Shepard, at the cost of 1000 Element Zero a pop.
*This terminal is somewhat confusingly named, given that Shepard has his own Advanced Training option in the Research Terminal which performs a different function.